The invention relates generally to a control method and apparatus for electromagnetic clutches and more specifically to a control method and apparatus wherein a random noise signal is superimposed upon a clutch engagement signal to provide improved engagement characteristics of the electromagnetic clutch.
Amongst motor vehicle manufacturers, two areas of vehicle performance are foci of constant study, engineering design and product improvement. These two areas can broadly be denominated vehicle operation and occupant comfort. Occupant comfort typically includes seating and other tactile and aesthetic considerations but also encompasses aspects of vehicle operation. Within the context of operation occupant comfort generally addresses noise, vibration and harshness (NVH) issues. These design criteria are nowhere more subjects of concern than in adaptive four-wheel drive vehicles where the complex power train, torque distribution configuration, control strategy and overall system operation create their own performance and control issues.
In adaptive four-wheel drive systems which operate under most driving conditions as two-wheel drive systems and automatically shift or select four-wheel drive when certain operating conditions such as wheel slip or other operating anomalies are sensed, the engagement of a clutch between the full-time or primary drive line and the part-time or secondary drive line to transfer drive energy to the secondary drive line must be both carefully controlled and preferably unnoticeable to the vehicle driver. Competing with this design goal of transparent and smooth clutch engagement is often the design preference to engage the clutch as quickly as possible in order to achieve vehicle control and slip regulation.
Furthermore, as the clutch engages and the coefficient of friction of the interleaved plates changes between static and dynamic values and operating modes, its torque throughput will be non-linear, i.e., not directly related to clutch coil current. Nonetheless, mechanical performance of a clutch must be smooth, well-modulated and without grabbing or other engagement phenomena which may be sensed by the vehicle driver or occupants. The present invention is directed to a control method and apparatus which improves engagement smoothness and overall performance of friction clutch packs in motor vehicle drive lines.
Engagement characteristics of the electromagnetic friction clutch pack assemblies disposed in, for example, motor vehicle transfer cases are improved, especially at the transition between static and dynamic friction modes, by superimposing a random noise signal on an otherwise smooth analog, stepped, incremental or digital signal. The controller and driver for the clutch includes a random noise (signal) generator having an output which is superimposed on the controller output generated in response to signals from various sensors disposed on the vehicle. When the controller inputs cause clutch engagement in accordance with the controller software, an engagement signal comprising both a drive portion and a smaller, random noise portion is supplied to the electromagnetic clutch operator. Improved, less perceptible clutch engagement action, especially at the transition between static and dynamic friction modes, is thereby achieved.
Thus it is an object of the present invention to provide a controller for an electromagnetic clutch having improved engagement characteristics.
It is a further object of the present invention to provide a method for controlling an electromagnetic clutch of a motor vehicle having improved engagement characteristics.
It is a still further object of the present invention to provide a method and apparatus for providing improved clutch engagement especially at the transition between static and dynamic friction modes.
It is a still further object of the present invention to provide a method of operating an electromagnetic clutch wherein a random noise signal is superimposed upon a control output thereby providing improved clutch engagement characteristics.
It is a still further object of the present invention to provide a controller for an electromagnetic clutch disposed in a transfer case or axle of a motor vehicle which provides a random noise signal superimposed upon a drive signal to improve clutch engagement characteristics.